U.S. patent application number 14/222312 was filed with the patent office on 2014-09-25 for electrochemical energy storage devices and components.
This patent application is currently assigned to Sila Nanotechnologies Inc.. The applicant listed for this patent is Sila Nanotechnologies Inc.. Invention is credited to Eugene Michael BERDICHEVSKY, Gleb YUSHIN, Bogdan ZDYRKO.
Application Number | 20140287301 14/222312 |
Document ID | / |
Family ID | 51569363 |
Filed Date | 2014-09-25 |
United States Patent
Application |
20140287301 |
Kind Code |
A1 |
YUSHIN; Gleb ; et
al. |
September 25, 2014 |
ELECTROCHEMICAL ENERGY STORAGE DEVICES AND COMPONENTS
Abstract
A battery electrode composition is provided comprising anode and
cathode electrodes and an electrolyte ionically coupling the anode
and the cathode. At least one of the electrodes may comprise a
plurality of active material particles provided to store and
release ions during battery operation. The electrolyte may comprise
an aqueous metal-ion electrolyte ionically interconnecting the
active material particles. Further, the plurality of active
material particles may comprise a conformal, metal-ion permeable
coating at the interface between the active material particles and
the aqueous metal-ion electrolyte. The conformal, metal-ion
permeable coating impedes water decomposition at the aforesaid at
least one of the electrodes.
Inventors: |
YUSHIN; Gleb; (Atlanta,
GA) ; ZDYRKO; Bogdan; (Atlanta, GA) ;
BERDICHEVSKY; Eugene Michael; (Atlanta, GA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Sila Nanotechnologies Inc. |
Atlanta |
GA |
US |
|
|
Assignee: |
Sila Nanotechnologies Inc.
Atlanta
GA
|
Family ID: |
51569363 |
Appl. No.: |
14/222312 |
Filed: |
March 21, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61804166 |
Mar 21, 2013 |
|
|
|
61832114 |
Jun 6, 2013 |
|
|
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Current U.S.
Class: |
429/188 |
Current CPC
Class: |
H01M 4/366 20130101;
H01M 2300/0002 20130101; Y02E 60/10 20130101; H01M 4/525 20130101;
H01M 10/4235 20130101; H01M 4/505 20130101; H01M 10/36
20130101 |
Class at
Publication: |
429/188 |
International
Class: |
H01M 4/36 20060101
H01M004/36 |
Claims
1. A metal-ion battery composition, comprising: anode and cathode
electrodes, wherein at least one of the electrodes comprises a
plurality of active material particles provided to store and
release ions during battery operation; and an electrolyte ionically
coupling the anode and the cathode, wherein the electrolyte
comprises an aqueous metal-ion electrolyte ionically
interconnecting the active material particles, wherein the
plurality of active material particles comprises a conformal,
metal-ion permeable coating at the interface between the active
material particles and the aqueous metal-ion electrolyte, whereby
the conformal, metal-ion permeable coating impedes water
decomposition at the at least one of the electrodes.
2. The metal-ion battery composition of claim 1, wherein the
conformal, metal-ion permeable coating has an average thickness is
in the range of about 10 nm to about 500 nm.
3. The metal-ion battery composition of claim 1, wherein the
conformal, metal-ion permeable coating encases each of the active
material particles individually.
4. The metal-ion battery composition of claim 1, wherein the
conformal, metal-ion permeable coating encases the plurality of
active material particles as a whole.
5. The metal-ion battery composition of claim 1, wherein the
conformal, metal-ion permeable coating has a non-uniform
composition that changes gradually with radial distance.
6. The metal-ion battery composition of claim 1, wherein the
conformal, metal-ion permeable coating is a composite coating
comprising a plurality of layers.
7. The metal-ion battery composition of claim 6, wherein the
plurality of layers comprises an outer layer formed from an
electrical insulator material for preventing electrochemical
reduction of the aqueous metal-ion electrolyte on the anode or
preventing electrochemical oxidation of the aqueous metal-ion
electrolyte on the cathode by accommodating a portion of the
voltage drop between the anode and cathode and thereby reducing the
voltage drop across the aqueous metal ion electrolyte.
8. The metal-ion battery composition of claim 6, wherein the
plurality of layers comprises a layer selected from the group
consisting of: an electrically conductive layer for electrically
connecting the active material particles; an interfacing layer for
enhancing uniformity or adhesion of another layer; a mechanically
stable layer for enhancing mechanical stability of the conformal,
metal-ion permeable coating; and a supplemental protection layer
for preventing electrochemical reduction of the aqueous metal-ion
electrolyte on the anode or preventing electrochemical oxidation of
the aqueous metal-ion electrolyte on the cathode.
9. The metal-ion battery composition of claim 1, wherein the
conformal, metal-ion permeable coating comprises, as a single or
outer layer, a chemically-linked, polymeric coating containing one
or more pH-regulating functional groups.
10. The metal-ion battery composition of claim 9, wherein the one
or more pH-regulating functional groups comprise an acidic
functional group for decreasing the pH of active particles at the
cathode to prevent electrochemical oxidation of the aqueous
metal-ion electrolyte.
11. The metal-ion battery composition of claim 9, wherein the one
or more pH-regulating functional groups comprise a basic functional
group for increasing the pH of active particles at the anode to
prevent electrochemical reduction of the aqueous metal-ion
electrolyte.
12. The metal-ion battery composition of claim 9, wherein the one
or more pH-regulating functional groups are borne by one or more
polymers attached to the surface of the active material
particles.
13. The metal-ion battery composition of claim 1, wherein the
conformal, metal-ion permeable coating is formed on the at least
one of the electrodes prior to a formation cycle of a cell
comprising the battery composition.
14. The metal-ion battery composition of claim 1, wherein the
conformal, metal-ion permeable coating is at least partially formed
on the at least one of the electrodes by decomposition of one or
more additives to the aqueous metal-ion electrolyte during a
formation cycle of a cell comprising the battery composition.
15. The metal-ion battery composition of claim 1, wherein the
conformal, metal-ion permeable coating comprises (i) a carbon or
(ii) one or more metals that enhance over-potential for water
decomposition by at least 0.25 V.
16. The metal-ion battery composition of claim 1, wherein the
conformal, metal-ion permeable coating comprises a plurality of
pores having an average pore size in the range of about 0.5 nm to
about 10 nm.
17. The metal-ion battery composition of claim 1, wherein the
active material particles are composites with a core-shell
structure.
18. The metal-ion battery composition of claim 17, wherein the core
of each active material particle is a nanocomposite comprising
active material and at least one of (i) pores, (ii) a carbon
additive, or (iii) a carbon scaffolding matrix.
19. The metal-ion battery composition of claim 18, wherein the
carbon scaffolding matrix is porous with an average pore size in
the range of about 0.5 nm to about 20 nm, and wherein the carbon
scaffolding matrix contains active material that at least partially
fills the pores.
20. The metal-ion battery composition of claim 17, wherein the
shell of each active material particle is a nanocomposite
comprising at least one of (i) an electrical conductive layer, (ii)
a mechanical stability layer, or (iii) a water impermeability
layer.
21. The metal-ion battery composition of claim 1, wherein the
metal-ion battery corresponds to an aqueous Li-ion battery.
Description
CLAIM OF PRIORITY UNDER 35 U.S.C. .sctn.119
[0001] The present application for patent claims priority to
Provisional Application No. 61/804,166 entitled "ELECTROCHEMICAL
ENERGY STORAGE DEVICES AND COMPONENTS" filed on Mar. 21, 2013, and
to Provisional Application No. 61/832,114 entitled "ELECTROCHEMICAL
ENERGY STORAGE DEVICES AND COMPONENTS" filed on Jun. 6, 2013, which
are expressly incorporated by reference herein.
BACKGROUND
[0002] 1. Field
[0003] The present disclosure relates generally to energy storage
devices, and more particularly to metal-ion battery technology and
the like.
[0004] 2. Background
[0005] Among the metal-ion batteries, Li-ion battery technology has
achieved the greatest commercial success, owing to the very high
gravimetric capacity (3860 mAh/g) and moderately high volumetric
capacity (2061 Ah/L) of Li anodes combined with the high activity
of Li and the high mobility of Li ions in various hosts.
[0006] Yet, other metal-ion batteries may also offer reasonably
high volumetric and gravimetric energy densities. For example, the
gravimetric specific capacity of Al (2980 mAh/g, calculated based
on the three-electron oxidation of Al) is close to that of Li,
while its volumetric storage capacity (8043 Ah/L) is four times
higher than that of Li, due to the fivefold higher density of Al.
The excellent storage capacity of Al combined with its broad
availability (Al is the most abundant metal in the Earth's crust,
contributing to over 8% of the total mass) and low cost, makes it
an attractive anode material. Similarly, Mg, for example, is nearly
as abundant as Al, but it is more active than Al and has high
gravimetric (2233 mAh/g) and volumetric (3885 Ah/L) specific
storage capacities. Na-ion and Ca-ion batteries may also offer some
advantages in selected applications. Finally, batteries that
combine metal cations and non-metal anions may also be utilized in
various applications.
[0007] Unfortunately, current Li-ion battery technology utilized
for transportation, grid storage, and electronic device fields is
expensive, slow, and unsafe. Such cells utilize organic
electrolytes and suffer from several limitations. Formation of Li
dendrites in commercial batteries is particularly challenging to
detect and prevent. When formed, they may lead to internal shorts,
which give rise to local heating, melting of the separator, thermal
runaway, and eventually fire. The high flammability of organic
electrolytes does not help this situation. In addition,
decomposition of organic electrolytes with the presence of water
and other impurities limit the cycle life of Li-ion batteries and
make their assembling expensive. Further, the relatively low ionic
conductivity of organic electrolytes combined with the low ionic
conductivity of the solid electrolyte interphase (SEI) limits the
power performance of Li-ion batteries.
[0008] The use of aqueous chemistry may significantly improve the
safety of Li-ion battery technologies, and, at the same time,
reduce the cost of Li-ion cells and corresponding battery packs.
However, the use of aqueous electrolytes is known to typically
limit the maximum voltage of aqueous Li-ion and other metal-ion
batteries to below around 1.2-1.5V. This low voltage limits the
energy density of the cells. In addition, the electrode fabrication
and cell construction developed for conventional Li-ion chemistry
utilizing organic electrolytes is very expensive. Adoption of
similar manufacturing technology for aqueous Li-ion cells will
increase their manufacturing cost.
[0009] Accordingly, there remains a need for improved aqueous
metal-ion batteries, components, and other related materials and
manufacturing processes.
SUMMARY
[0010] Embodiments disclosed herein address the above-stated needs
by providing improved aqueous metal-ion (such as Li-ion) battery
components, improved batteries made therefrom, and methods of
making and using the same. Such aqueous metal-ion batteries
facilitate the incorporation of advanced material synthesis and
electrode fabrication technologies, and enable fabrication of high
voltage and high capacity aqueous metal-ion batteries at a cost
lower than that of conventional Li-ion battery technology.
[0011] A battery electrode composition is provided comprising anode
and cathode electrodes and an electrolyte ionically coupling the
anode and the cathode. At least one of the electrodes may comprise
a plurality of active material particles provided to store and
release ions during battery operation. The electrolyte may comprise
an aqueous metal-ion electrolyte ionically interconnecting the
active material particles. Further, the plurality of active
material particles may comprise a conformal, metal-ion permeable
coating at the interface between the active material particles and
the aqueous metal-ion electrolyte. The conformal, metal-ion
permeable coating impedes water decomposition at the aforesaid at
least one of the electrodes.
[0012] The conformal, metal-ion permeable coating may have an
average thickness is in the range of about 10 nm to about 500 nm.
The conformal, metal-ion permeable coating may encase each of the
active material particles individually. Alternatively or in
addition, the conformal, metal-ion permeable coating may encase the
plurality of active material particles as a whole. In some designs,
the conformal, metal-ion permeable coating may be generally
uniform, while in other designs it may have a non-uniform
composition that changes gradually with radial distance (e.g., from
an inner surface to an outer surface). In this case, a more
chemically and mechanically robust coating may be formed.
[0013] In various embodiments, the conformal, metal-ion permeable
coating may be a composite coating comprising a plurality of
layers. For example, the plurality of layers may comprise an outer
layer formed from an electrical insulator material for preventing
electrochemical reduction of the aqueous metal-ion electrolyte on
the anode or preventing electrochemical oxidation of the aqueous
metal-ion electrolyte on the cathode. This may be achieved by the
insulative outer layer accommodating a portion of the voltage drop
between the anode and cathode, thereby reducing the voltage drop
across the aqueous metal ion electrolyte. In other examples, the
plurality of layers may comprise an electrically conductive layer
for electrically connecting the active material particles, an
interfacing layer for enhancing uniformity or adhesion of another
layer, a mechanically stable layer for enhancing mechanical
stability of the conformal, metal-ion permeable coating, or a
supplemental protection layer for preventing electrochemical
reduction of the aqueous metal-ion electrolyte on the anode or
preventing electrochemical oxidation of the aqueous metal-ion
electrolyte on the cathode.
[0014] In some applications, the conformal, metal-ion permeable
coating may comprise, as a single or outer layer, a
chemically-linked, polymeric coating containing one or more
pH-regulating functional groups. As an example, the one or more
pH-regulating functional groups may comprise an acidic functional
group for decreasing the pH (e.g., to pH of approximately 4 or
below) of active particles at the cathode to prevent
electrochemical oxidation of the aqueous metal-ion electrolyte. As
another example, the one or more pH-regulating functional groups
may comprise a basic functional group for increasing the pH (e.g.,
to pH of approximately 9 or above) of active particles at the anode
to prevent electrochemical reduction of the aqueous metal-ion
electrolyte. In either case, the one or more pH-regulating
functional groups may be borne by one or more polymers attached to
the surface of the active material particles.
[0015] In some designs, the conformal, metal-ion permeable coating
may be formed on the aforesaid at least one of the electrodes prior
to a formation cycle of a cell comprising the battery composition,
while in other designs it may be at least partially formed on the
aforesaid at least one of the electrodes by decomposition of one or
more additives to the aqueous metal-ion electrolyte during a
formation cycle of a cell comprising the battery composition. That
is, the coating layer(s) can be deposited on the electrode surface
either (1) prior to assembling of the cell or (2) formed in-situ
during the so-called formation cycle(s) of the cell when
additive(s) to an aqueous electrolyte decompose at a potential
where water does not yet decompose, thus forming a protective
coating on the electrode surface, or (3) both.
[0016] In different designs, the conformal, metal-ion permeable
coating may comprise (i) a carbon or (ii) one or more metals that
enhance over-potential for water decomposition by at least 0.25 V.
The conformal, metal-ion permeable coating may also comprise a
plurality of pores having an average pore size in the range of
about 0.5 nm to about 10 nm.
[0017] The active material particles may be composites with a
core-shell structure. The core of each active material particle may
be, for example, a nanocomposite comprising active material and at
least one of (i) pores, (ii) a carbon additive, or (iii) a carbon
scaffolding matrix. The carbon scaffolding matrix may be porous
with an average pore size in the range of about 0.5 nm to about 20
nm, and may contain active material that at least partially fills
the pores. The shell of each active material particle may be, for
example, a nanocomposite comprising at least one of (i) an
electrical conductive layer, (ii) a mechanical stability layer, or
(iii) a water impermeability layer.
[0018] In general, the metal-ion battery may correspond to an
aqueous Li-ion battery, or other such aqueous metal-ion
batteries.
[0019] Various methods of fabricating a battery electrode
composition comprising active particles are also provided. They may
comprise, for example: providing active material particles to store
and release ions during battery operation; electrically connecting
the active particles with a current collector; forming a conformal
protective coating on the electrode surface in such a way that the
electrode remains porous while all (or at least a substantial
portion) of its open pore surface area is covered with such a
coating. For connecting the active particle together during the
electrode fabrication, the method may involve mixing the active
particles with a binder and annealing at an elevated temperature to
cause solidification of the bonded particles in a particular shape.
In some embodiments, the surface of the active particles may allow
sintering particles together at elevated temperatures and thus not
require a binder.
[0020] In some embodiments, the shape of the produced electrodes
may be planar (for sandwich-type electrode stacking with a
separator layer in between positive and negative electrodes). In
other embodiments, the shape of the produced electrodes may be
cylindrical (for cylindrical cell fabrication with a cylindrical
electrode in another hollow cylinder electrode and a cylindrical
separator layer in between cylindrical positive and negative
electrodes).
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The accompanying drawings are presented to aid in the
description of embodiments of the invention and are provided solely
for illustration of the embodiments and not limitation thereof.
[0022] FIG. 1 illustrates a stability profile for water (H.sub.2O)
across pH.
[0023] FIG. 2 illustrates an electrochemical cell design for
localizing pH at the electrodes to enhance the aqueous electrolyte
stability voltage range.
[0024] FIG. 3 provides examples of various pH shifting and chemical
bonding groups that may be used in conjunction with the cell design
of FIG. 2.
[0025] FIG. 4 is a schematic view of polymer chain adsorption of a
pH-modifying coating on an electrode substrate surface.
[0026] FIG. 5 provides two graphs illustrating the impact of
pH-regulating coatings on the electrochemical stability of a
pH-neutral aqueous electrolyte.
[0027] FIG. 6 is a cross-sectional view of an electrode
illustrating the use of an electrically insulative but ionicially
conductive conformal coating.
[0028] FIG. 7 illustrates the voltage drop between the anode and
the cathode of an aqueous cell with and without a protective
coating of the type shown in FIG. 6.
[0029] FIGS. 8-10 are schematic illustrations of different examples
of in-situ formation of the protective coating layer on an
electrode via different suitable precursors.
[0030] FIG. 11 illustrates an example multi-layer implementation of
the protective coating layer impeding aqueous electrolyte
decomposition.
[0031] FIG. 12 is a cross-section view of different example
particle designs incorporating one or more Li-ion permeable, but
solvent impermeable protective shell(s).
[0032] FIG. 13 provides an example of a high capacity aqueous
Li-ion battery with a pH-modified anode and cathode.
[0033] FIG. 14 provides an example of different porous particle
designs containing a conversion-type active material (sulfur) that
experiences volume changes upon Li insertion.
[0034] FIG. 15 is a flow chart illustrating an example method of
fabricating a battery electrode composition comprising active
particles.
[0035] FIG. 16 shows a comparison of two cell constructions,
including a conventional Li-ion cell side by side an aqueous Li-ion
cell as described herein.
[0036] FIG. 17 shows select performance characteristics of the two
cell constructions, including a conventional Li-ion cell side by
side an aqueous Li-ion cell as described herein.
DETAILED DESCRIPTION
[0037] Aspects of the present invention are disclosed in the
following description and related drawings directed to specific
embodiments of the invention. The term "embodiments of the
invention" does not require that all embodiments of the invention
include the discussed feature, advantage, process, or mode of
operation, and alternate embodiments may be devised without
departing from the scope of the invention. Additionally, well-known
elements of the invention may not be described in detail or may be
omitted so as not to obscure other, more relevant details.
[0038] Aqueous metal-ion (such as Li-ion) technology may offer
enhanced safety, enhanced power performance and reduced cost
compared to traditional Li-ion technology that utilizes organic
electrolyte(s). Organic electrolytes used in conventional Li-ion
batteries exhibit specific Li-ion conductance of up to about 3
mS/cm. In contrast, Li ions in aqueous solutions exhibit
conductance of about 75 mS/cm. Thus, for the same electrodes and
current rate, organic electrolytes may induce about a twenty-five
times higher polarization. Therefore, Li-ion battery cells with
aqueous electrolyte(s) may operate at more than an order of
magnitude higher current densities and accordingly provide an order
of magnitude higher power. Conversely, for the same power
performance, aqueous Li-ion batteries may utilize thicker
electrodes.
[0039] The key bottlenecks in the development of stable, low-cost,
aqueous Li-ion technology, however, include: (i) a low
thermodynamically stable voltage range for aqueous electrolytes;
(ii) the absence of stable electrode materials that offer high
capacity; and (iii) high cost and poor compatibility of traditional
Li-ion cell manufacturing techniques with aqueous Li-ion
technologies.
[0040] The improvements in aqueous Li-ion battery technology
described herein address the above-noted challenges, and may be
implemented via one or more of several complimentary techniques,
including but not limited to: (1) different techniques for
increasing the voltage stability range of pH-neutral aqueous
electrolytes by forming ion-permeable coatings on the electrode
surface that impede aqueous electrolyte decomposition as well as
the resulting gas generation and self-discharge; (2) different
techniques for reducing the cost of electrode fabrication and
aqueous cell assembling; and (3) different techniques for forming
advanced nanostructured high-capacity electrodes compatible with
aqueous chemistry.
[0041] In the description below, several examples are provided in
the context of aqueous Li-ion batteries because of the current
prevalence and popularity of Li-ion technology. However, it will be
appreciated that such examples are provided merely to aid in the
understanding and illustration of the underlying techniques, and
that these techniques may be similarly applied to various other
metal-ion batteries, such as aqueous Na-ion, aqueous Ca-ion,
aqueous K-ion, aqueous Mg-ion, and other aqueous metal-ion
batteries.
[0042] In addition, various aspects of the present disclosure may
be applied to various aqueous electrochemical capacitors, aqueous
pseudocapacitors, aqueous Li-ion capacitors, aqueous asymmetric
supercapacitors, hybrid electrochemical capacitor-battery devices
(where one of the electrodes is battery-like, while the other is
electrochemical capacitor-like), and other aqueous electrochemical
energy storage devices in order to enhance their performance (for
example, to enhance maximum charge voltage or to reduce leakage
current, or both). Further, various aspects of the present
disclosure may also be applied to electrochemical energy storage
devices based on non-aqueous electrolytes.
[0043] According to different embodiments, various aspects of the
present disclosure may be applied to both the positive electrode
and the negative electrode of aqueous electrochemical energy
storage devices, or to the electrodes individually (either the
positive electrode or the negative electrode). Application to only
one of the electrodes may be used to prevent aqueous electrolyte
decomposition on such an electrode. For example, application to a
cathode in particular may help prevent oxygen evolution at higher
potentials. Application to an anode in particular may help prevent
hydrogen evolution at lower potentials.
[0044] Several methods are described below to enhance the aqueous
electrolyte stability voltage range. For example, in a first
method, a pH modification of the electrode surface may be
implemented. This may be particularly beneficial for pH-neutral
aqueous electrolytes. In a second method, a conformal coating may
be formed on the electrode surface to account for some of the
voltage drop between the electrodes, allowing liquid electrolyte to
be maintained within a stable potential range. This may be
generally applied to electrolytes other than pH-neutral aqueous
electrolytes.
[0045] FIG. 1 illustrates a stability profile for water (H.sub.2O)
across pH. As shown, at high potentials, H.sub.2O decomposes with
O.sub.2 evolution, and at low potentials, with H.sub.2 evolution.
The potential of H.sub.2O oxidation at the cathode, 2
H.sub.2O.fwdarw.O.sub.2(g)+4H.sup.++2e.sup.-, is governed by the
Nernst equation and can be increased to above 1.2 V (vs. NHE) at
low pH values. Similarly, the potential of H.sub.2O reduction at
the anode, 2 H.sup.++2e.sup.-.fwdarw.H.sub.2(g) or
H.sub.2O+2e.sup.-.fwdarw.H.sub.2(g)+2OH.sup.-, can be reduced to
below -1 V (vs. NHE) at high pH values.
[0046] FIG. 2 illustrates an electrochemical cell design for
localizing pH at the electrodes to enhance the aqueous electrolyte
stability voltage range. In this design, the surfaces of both
electrodes, including an anode 202 and a cathode 204, are
functionalized with pH-tuning moieties 203, 205 of macromolecules
without changing the pH in the bulk of a pH-neutral aqueous Li-ion
electrolyte solution 206 (such as solutions of Li.sub.2SO.sub.4,
LiCl, LiNO.sub.3, or other Li salts in H.sub.2O).
[0047] This design is advantageous in that the local pH value can
be independently adjusted at the surface of each electrode via the
pH-tuning moieties confined to the surfaces of corresponding
electrodes. Polymers or macromolecules bearing these functional
moieties have been found to be particularly useful for this
purpose. Such macromolecules can be physically or chemically
attached to the surface of the electrode material, affecting the pH
value only locally, without changing the pH in the bulk of the
battery electrolyte solution (as is further illustrated in
corresponding average pH distribution shown in FIG. 2).
[0048] FIG. 3 provides examples of various pH shifting and chemical
bonding groups that may be used in conjunction with the cell design
of FIG. 2. The decrease of pH in the vicinity of the electrode can
be achieved by attaching polymer-bearing acidic groups, such as
carboxylic, phosphoric, or sulfuric moieties. For simplicity, such
polymers may be referred to as "acidic." Depending on the pKa of
the acidic group in the polymer, the local pH value can be tuned in
wide ranges from about pH=6 to about pH=0. Among the
above-mentioned acids, sulfuric acid is the strongest (with pKa
less than 1), thus providing the largest local pH shift. In order
to shift the pH near the battery electrodes into basic conditions,
polymers bearing amine moieties in their structure can be used. For
simplicity, such polymers may be referred to as "basic." Depending
on the pKa of the amine used, the local pH values can be varied
from about pH=7 to about pH=12. Another way to increase the local
pH is to bind a weak acid polyanion (polymer containing weak acidic
groups) salt of a strong base. In the case of an aqueous Li-ion
battery, the choice of a strong base is predefined to be LiOH. Due
to hydrolysis, salts formed from a strong base and a weak acid will
increase pH locally.
[0049] Long-term stability of the pH-modifying coatings may be
enhanced by chemical bonding to the particle surface and/or coating
cross-linking (e.g., via the chemical bonding groups shown in FIG.
3). To obtain a pH modifying polymer capable of chemically bonding
to the electrode surface, two monomers may be co-polymerized. One
co-monomer may bear a pH-modifying group. The second co-monomer may
contain in its structure a chemical group capable of forming
covalent bonds with particles of active materials. By changing the
ratio between the two co-monomers, the bonding and pH tuning
properties of the polymer coating can be tuned for more optimized
electrode performance.
[0050] FIG. 4 is a schematic view of polymer chain adsorption of a
pH-modifying coating on an electrode substrate surface. The
polymeric chemistry of the pH-modifying coating will provide
long-term stability. When polymers are adsorbed, they form multiple
contacts with the surface 402 called "trains" 404, as shown. "Loop"
sections 406 and "tail" sections 408 are not connected to the
substrate. On average, the train fraction for relatively high
molecular weight polymers adsorbed on the surface may be about
0.15-0.25 and 3-4 monomeric units may be involved in each train
section. This means that a polymer chain with a typical degree of
polymerization N=1,000 has at least (0.15)*(3)*(1000)=450 contacts
with the surface. Despite each contact being a relatively weak
bond, the large number of the contacts results in very strong
interaction between the surface and the polymer molecule, often
exceeding the strength of covalent bonds.
[0051] The pH-tuning polymers may be utilized as an additional
surface coating on the surface of metal-ion battery electrodes or
as binders used in the preparation of battery electrodes. Epoxy
groups have been found to be particularly suitable for permanent
chemical attachments to various surfaces. In particular, they can
react with metal oxides to form chemical bonds between metal oxide
and pH-modifying polymers. Similarly, epoxy groups are capable of
binding with functionalities intrinsically present on carbon
surfaces, such as carboxyl groups. In order to induce cross-linking
of the polymer coatings, carbon-carbon double bonds within the
polymer structure can be utilized.
[0052] In some applications, it may be important for the produced
coatings to remain permeable to electrolyte solvent (such as water
in the case of aqueous metal-ion batteries). The pH-modifying units
located in the "loops" and "tail" sections of the attached polymer
coating are not linked to the surface. These groups are polar, and,
therefore, are easily hydrated by water molecules providing both
the required pH shift and, equally important, channels for easy
active ion migration in and out of the active electrode material.
From the ratio of "trains" to "loops" (e.g., 0.15-0.25 to
0.85-0.75) in the polymer macromolecule coatings, it can be
estimated that about 75-85% of the particle surface will have free
access for active ions. Therefore, formation of the polymer
coatings may have a very minor effect on the power characteristics
of the aqueous metal-ion batteries.
[0053] FIG. 5 provides two graphs illustrating the impact of
pH-regulating coatings, on the surface of glassy carbon working
electrodes, on the electrochemical stability of a pH-neutral
aqueous electrolyte (1M LiCl) measured in a 3-electrode
configuration. On the left, it can be seen that the voltage
stability range is expanded to below -1.2 V vs. NHE by coating a
carbon surface with a polymer bearing basic moieties. On the right,
it can be seen that the voltage stability range is expanded to over
1.5 V vs. NHE by coating a carbon surface with a polymer bearing
acidic functional moieties. The higher current observed for the
carbon surface with a polymer bearing acidic functional moieties is
likely related to the pseudo-capacitance induced by the acidic
functional groups of the polymer coating.
[0054] It will be appreciated that pH-modifying coatings may be
deposited directly on the surface of active particles or on the
surface of another layer that coats the active particles and may
additionally serve various other functions, such as, for example,
additionally prevent water decomposition on the electrode surface,
prevent degradation of active material, improve electrical
conductivity within the electrode, or improve the interface between
the active particles and the pH-modifying coating, to name a
few.
[0055] As discussed above, it will be appreciated that pH-modifying
coatings may be used for other chemistries of anodes and cathodes
as well as for electrochemical capacitor applications and hybrid
devices.
[0056] In some applications, in order to further minimize H.sub.2
evolution, anodes may additionally be provisioned with microporous
and mesoporous additives, capable of proton and H.sub.3O.sup.+
adsorption, and known to prevent water decomposition at low
potentials. Such additives may be provided in the form of a coating
around the active particles (or electrode) or in the form of
individual particles, or even in the form of electrolyte
additives.
[0057] In some configurations, electrolyte additives may be used to
create oxide/hydroxide coatings with a basic nature deposited on
top of the electrodes. For example, in the case of metal nitrates
as additives to the battery electrolyte, metal oxide/hydroxide
coatings can be formed during electro-reduction at the electrode.
Ions of, for example, Mg.sup.2+, Al.sup.3+, Cr.sup.3+, Fe.sup.3+,
Mn.sup.3+, and Co.sup.2+, can be reduced during the process.
However, this approach may not be applicable to nitrates of metals
such as Cu, Tl, Bi, and Pb, and yields only metal deposits.
Utilization of perchlorate salts of Cu, Tl, Bi, and Pb results in
hydroxide/oxide film formation during electro-reduction.
[0058] Another method for synthesizing oxide films is metal-ion
galvanostatic reduction in the presence of hydrogen peroxide.
Coatings consisting of ZrO.sub.2, Al.sub.2O.sub.3,
Al.sub.2O.sub.3--ZrO.sub.2, and Al.sub.2O.sub.3--Cr.sub.2O.sub.3
can be made by this approach.
[0059] Oxide coatings on the battery electrode can be obtained, for
example, by a two-step process. In the first step, a metal coating
may be deposited on the electrode by electroplating. In the second
step, the metal coating may be converted into an oxide by
electro-oxidation. Oxides of the metal, which can be
electrodeposited from aqueous solutions, can be deposited in this
way.
[0060] The desired coating porosity and enhanced proton adsorption
can be achieved by gentle heat treatment in the case of hydroxide
coatings. Heat treatment leads to partial dehydration of the
coating, creating porosity. A deposition regime (e.g.,
galvanostatic, potentiostatic, current pulsing, or voltage pulsing)
can also be utilized for the creation of microporosity in the
coatings.
[0061] In some applications, porous metal (or porous carbon)
coatings or porous metal (or porous carbon) powder may efficiently
prevent H.sub.2 evolution on the anode or O.sub.2 evolution on the
cathode. Several metals are known to offer high over-potentials for
H.sub.2 and O.sub.2 evolution, and have been found to be useful as
additives for aqueous Li-ion batteries. For example, iron (Fe)
increases the potential of O.sub.2 generation at the cathode by
about 0.75 V, nickel (Ni) by 0.56 V, lead (Pb) by 0.81 V, and
graphite by 0.95V. Other metals, for example zinc, bismuth, and
mercury, also significantly increase the potential of water
decomposition at the cathode. All these materials can be used as
coatings or as a powder in cathode construction. Similarly, several
metals decrease the increase of H.sub.2 generation at the anode.
For example, graphite, lead, zinc, mercury, and bismuth lower the
potential of water decomposition and H.sub.2 evolution on the anode
by at least 0.6 V. All these materials can be used as coatings or
as a powder in cathode construction. In some configurations, the
presence of micropores and mesopores within such materials has been
found to further prevent water decomposition.
[0062] In some configurations (for example, when it is advantageous
to reduce the cost of the electrode fabrication or to increase the
electrode density), the coating of conductive carbon or selective
metal(s) may preferably be not porous.
[0063] FIG. 6 is a cross-sectional view of an electrode
illustrating the use of an electrically insulative but ionicially
conductive conformal coating. In this example, a thin protective
coating 602 is provided to cover the electrode surface via active
particles 604 electrically connected to a current collector 606. In
some applications, it may be advantageous to form such a conformal,
electrically insulative (i.e., essentially or substantially
impermeable to electrons) but ionically conductive (i.e.,
essentially or substantially permeable to ions participating in
energy storage) conformal coatings on the surface of electrodes for
aqueous meal-ion batteries.
[0064] Conventionally, the voltage between the anode and the
cathode of an aqueous cell is applied across an aqueous electrolyte
layer. When such a voltage exceeds some critical value (often in
the range of about 0.6 V to about 1.9 V) water decomposition takes
place with oxygen evolution on the cathode or hydrogen evolution on
the anode, or both. However, if one or both electrodes are coated
with a thin electrically insulative but ionically conductive
protective layer, this voltage drops across both the electrolyte
and the protective layer in series. This provides a particular
advantage for stabilizing an aqueous electrolyte against
decomposition.
[0065] FIG. 7 illustrates the voltage drop between the anode and
the cathode of an aqueous cell with and without a protective
coating of the type shown in FIG. 6. As shown, if, for example, the
total ionic (e.g., Li ion) resistance of this protective layer(s)
approximately equals the ionic resistance of the aqueous
electrolyte, the voltage drop across the aqueous electrolyte
becomes approximately half of the potential difference between the
anode and the cathode. If, for example, by using pH modifying
moieties on the surface of the protective layer, the stability
range of an aqueous electrolyte can approach 1.9 V, then the
maximum voltage between the anode and the cathode may safely
approach 3.8 V because half of that voltage will be dropped across
the protective layer. In this case, the voltage of such an aqueous
Li-ion cell, for example, approaches that of the conventional
Li-ion cell with an organic electrolyte. This high voltage
increases the energy density of the aqueous Li-ion cell, which is
particularly important for practical applications.
[0066] According to various embodiments, the overall ionic
resistance of the protective layer(s) can be adjusted to provide an
optimum combination of high total cell voltage, power performance,
and reliability. Further, the protective layer may be applied to an
anode, a cathode, or both. If applied to an anode, it may prevent
hydrogen evolution at low anode potentials. If applied to a
cathode, it may prevent oxygen evolution at high cathode
potentials.
[0067] In many applications, it may be advantageous for this
protective layer to uniformly coat the electrolyte-accessible
surface of the (porous) electrode. This is because non-uniformities
in the layer thickness may induce undesirable variations in the
resistivity of the protective layer. If some portion of the
protective layer becomes too thin in some area of the electrode,
the voltage drop across the aqueous electrolyte may exceed a
critical value leading to water decomposition. If some portion of
the protective layer becomes too thick in some area of the
electrode, it will impede the ion transport in this area, limiting
capacity utilization at high current densities. For practical
reasons, it may be desirable to have no more than a three-fold
variation in the thickness of the protective layer within the
protected electrode.
[0068] In some applications, it may be advantageous for the overall
coating thickness of the protective coating layer to range from
about 10 nm to about 500 nm. Thinner coatings may be prone to
defects. In some cases, coatings thinner than 5 nm may allow
quantum mechanical tunneling of the electrons, which is undesirable
as it will permit electrochemical reduction or oxidation of water
at extreme potentials and may prevent the protective coating from
function properly. Coatings thicker than 500 nm may impede ion
transport or contribute to a significant portion of the total mass
or volume, which may also be undesirable.
[0069] The ionic conductivity of the protective layer may be made
relatively low. For example, when the effective diffusion distance
of Li ions in the aqueous electrolyte is 1.6 mm, its ionic
resistance (per 1 cm.sup.2 area of the electrode) will be equal to
(0.16 cm)*(1/0.075 mS cm.sup.-1)=2.1 Ohm, assuming ionic
conductance of the aqueous Li electrolyte to be 75 mS/cm. By way of
example, consider a design in which the porous electrode surface
area is 100 times larger than the geometrical area of the electrode
(due to internal porosity) and that this surface is uniformly
coated with the protective layer. In this example, the thickness of
the protective layer is 20 nm and its resistance is set to 2.1 Ohm.
Accordingly, the Li ionic conductance of this layer will be a mere
(0.000002 cm)/(100)/(2.1 Ohm).apprxeq.10.sup.-8 S cm.sup.-1. When
the effective diffusion distance of Li ions in the aqueous
electrolyte is larger (e.g., 8 mm for example), the Li ionic
conductance of this layer must be even smaller, a mere
.apprxeq.10.sup.-9 S cm.sup.-1. This is a relatively low value, and
easy to achieve in many water-compatible ceramic and polymer
materials. It does not require development of water-compatible
highly conductive solid electrolytes.
[0070] The application of such conformal protective coating(s) on
the porous electrode surface provides several key advantages over,
for example, a thick solid conductive membrane layer that separates
the aqueous electrolyte from a solid nonporous electrode or a
porous electrode filled with a non-aqueous electrolyte. First, the
conformal protective coatings do not require high conductance for
providing high overall power performance. Second, in most cases,
these coatings are significantly less expensive to deposit because
their thicknesses are quite small and because they do not need to
possess high ionic conductance. Third, such coatings are more
resistant to failure because even if one particle fails (e.g., due
to a coating defect) and reacts with the electrolyte, the whole
cell can continue to function, losing only a tiny fraction of the
overall capacity. Furthermore, as discussed elsewhere herein, the
defect may be sealed or repaired during cycling by using additives
within the electrolyte. In contrast, the high conductivity thick
membranes (typically 10-500 microns) that may, in principle, also
be used, suffer from high prices that make them uncompetitive and
low conductivity that fail to provide high power performance. More
importantly, if a large defect develops within such a membrane, it
may ruin the entire cell because the individual particles are not
protected.
[0071] Formation of the insulative but ionically conductive
protective layer conformal coatings on the electrode surface can be
performed via electro-reduction (on the anode) or electro-oxidation
(on the cathode) of ceramic precursors dissolved in aqueous
electrolyte. For example, electro-reduction of the metal ions on
the anode can be used to synthesize a variety of metal hydroxide or
oxide films. The oxide formation instead of Me electro-deposition
can be achieved by bath composition. For example, metal nitrates
will yield hydroxide (oxide) films. Examples include, but are not
limited to, ions of Mg.sup.2+, Al.sup.3+, Cr.sup.3+, Fe.sup.3+,
Mn.sup.3+, and Co.sup.2+. However, salts of Cu, Tl, Bi, and Pb
yield only metal deposits in the case of nitrate counter ions.
Utilization of perchlorate salts of Cu, Tl, Bi, or Pb results in
hydroxide (oxide) formation during electro-reduction.
[0072] Another method for synthesizing oxide films is galvanostatic
reduction in the presence of hydrogen peroxide. Coatings consisting
of ZrO.sub.2, Al.sub.2O.sub.3, Al.sub.2O.sub.3--ZrO.sub.2, and
Al.sub.2O.sub.3--Cr.sub.2O.sub.3 can be made by this approach.
[0073] Oxide coatings on the battery electrode can be obtained, for
example, by a two-step process. In the first step, a metal coating
is made by electroplating. In the second step, the metal coating is
converted into oxide by electro-oxidation. Oxides of the metal,
which can be electrodeposited from aqueous solutions, can be
deposited in this way.
[0074] Metal oxide/hydroxide films can be generated by oxidation at
the cathode. The pH of the electrolyte is chosen in such a way that
the lower oxidation state is stable while the higher oxidation
state readily undergoes hydrolysis to yield the metal oxide or
hydroxide. Examples include, but are not limited to, MnO.sub.2,
PbO.sub.2, V.sub.2O.sub.5, MnO(OH), and CoO(OH).
[0075] By fine-tuning the applied cell potentials, the oxidizing or
reducing power can be continuously varied and suitably selected.
Galvanostatic, potentiostatic, and cyclic voltammetry (CV) modes of
deposition or their combinations can be utilized for formation of
the coating with desired properties.
[0076] Formation of the insulative but ionically conductive
protective layer conformal coatings on the electrode surface may
also be performed via electro-grafting of monomers present in an
electrolyte solution. In this case, it is preferable that
electro-grafting takes place at potentials where the majority of
electrolyte solvent remains stable. In some applications, it may be
preferable for the electro-grafting to take place in-situ during
the first cycle of the aqueous metal-ion battery. In this case, a
monomer should be dissolved in this electrolyte aqueous solution.
In some applications, this electro-grafting may be employed as a
secondary safety measure; that is, if the pre-deposited coating
fails in some part of the electrode or in some part of an active
particle due to a manufacturing defect, this water decomposition
site will be neutralized by in-situ formation of the grafted
layer.
[0077] In one example, a vinyl monomer present in the electrolyte
solution may be used as a precursor for electro-grafting. Upon
battery charging, a negative potential applied to an anode will
cause reduction of the double bond of the vinyl monomer, causing
anion formation, which, in turn, will cause monomer polymerization
and grafting to the electrically conductive electrode (electron
conductive) or electrically conductive site(s) on the electrode
surface.
[0078] FIGS. 8-10 are schematic illustrations of different examples
of in-situ formation of the protective coating layer on an
electrode via different suitable precursors.
[0079] In one example, acrylonitrile may be electro-grafted on the
electrode surface, as shown schematically in FIG. 8. Via proper
design of the (meth)acrylate monomers, electro-grafting in water
media is also an option, as shown schematically in FIG. 9. Three
major structural features of the monomer have been found to be
advantageous in this regard: (i) a long hydrophobic alkyl chain
capable of expelling water from the electrical double layer of the
battery electrode and increasing the electrochemical window of the
aqueous electrolyte; (ii) the capping of this chain by a cationic
hydrophilic head at one end in order to trigger micellization and
desorption to the anode surface; and (iii) the capping of the
second chain-end by a polymerizable acrylic fragment.
[0080] Other examples of a suitable precursor for the in-situ
formation of the protective coating layer on an electrode (such as
the anode) are diazonium salts' derivatives. These molecules can be
cleaved when electro-reduced on the battery anode, as shown
schematically in FIG. 10. The radicals formed as a result of an
electron transfer from the conductive anode surface (or conductive
site on the anode surface) eventually induce formation of a
covalent bond with the electrode. Because the electro-grafted
molecules are neutral, no polyaddition reaction occurs (in contrast
to the electro-reduction of acrylic monomers). The nature of the
substituent R in the aromatic ring can be tuned in order to achieve
the desired ionic resistance of the coating layer.
[0081] Careful selection of the electro-grafting conditions (such
as reagent concentration, grafting potential, and, when grafting is
performed in a different cell, pH of the grafting solution) allows
for a stable surface layer formation with a desired morphology and
precise control of film thickness and ionic resistivity.
[0082] FIG. 11 illustrates an example multi-layer implementation of
the protective coating layer impeding aqueous electrolyte
decomposition. In this example, the multilayer coating structure
includes one or more inner layers 1102, one or more intermediate
layers 1104, and one or more outer layers 1106 disposed on or
around active particles 1108, although it will be appreciated that
the number and arrangement of the different layers may vary from
application to application as desired. Each of the layers may bear
different functions.
[0083] An inner layer may be deposited, for example, to assist in
electrically connecting active particles of the electrode. In this
case, this layer should be made electrically conductive. Examples
of materials for such a layer include but are not limited to a
conductive carbon coating or a conductive metal coating, which
should be stable in the potential range for the electrode of
interest. Nickel is an example of such a metal that is suitable for
some anodes.
[0084] An intermediate layer can also be deposited in order to
assist in forming uniform coating of any subsequent layers.
Examples of materials for such a layer include but are not limited
to metal(s), metal alloy(s), metal oxide(s), metal fluoride(s),
metal sulfide(s), various other ceramic coatings, polymer(s), and
composite(s), to name a few. It is desirable that this material
should also be stable in the potential range for the electrode of
interest and not undergo undesirable phase transformation
reactions.
[0085] Another intermediate layer can also be deposited in order to
enhance the mechanical properties of the overall coating or enhance
mechanical stability of individual particles. Examples of materials
for such a layer include but are not limited to carbon, metal(s),
metal alloy(s), metal oxide(s), metal fluoride(s), various other
ceramic coating(s), and composite(s), to name a few.
[0086] One or more outer layer(s) may be deposited to provide
additional protection against aqueous electrolyte decomposition or
other useful functions. Examples of materials for such a layer
include but are not limited to various metal(s) (as previously
described), metal oxide(s), metal fluoride(s), metal sulfide(s),
various other ceramic coatings, polymer(s) and composite(s), to
name a few. It is desirable that this material should also be
stable in the potential range for the electrode of interest and not
undergo undesirable phase transformation reactions.
[0087] All layers should be permeable to ion transport in order to
provide energy storage capability to the active particles. In some
applications, it may be preferred that at least one of the layers
does not allow electron transport, thus preventing electrochemical
reduction of the aqueous electrolyte on the anode or preventing
electrochemical oxidation of the aqueous electrolyte on the
cathode. In this case, an electrical insulator of sufficient
thickness (e.g., typically greater than about 5 nm) should be used
to prevent electron tunneling. This function should also be
maintained during cycling without forming electron conduction paths
by, for example, phase transformation or defect formation.
[0088] In some applications, it may be advantageous for the most
outer layer to contain pH-regulating moieties that change the local
pH in the vicinity of the electrode, thus assisting in preventing
aqueous electrolyte decomposition, as described in more detail
above.
[0089] In some applications, it may be beneficial for some of the
coating layer(s) to be deposited on the electrode surface prior to
assembling of the cell. In this case, high flexibility can be
achieved in both the chemistry and morphology of the layer(s). In
some applications, it may be beneficial for at least the outer
coating layer(s) to be formed in-situ during the so-called
formation cycle(s) of the cell when additive(s) to an aqueous
electrolyte decompose at a potential, where water does not yet
decompose, thus forming a protective coating on the electrode
surface. In this case, the overall cost of the cell fabrication can
be reduced. In some applications (for example, when multiple
protection mechanisms are desired), the coating layer(s) may be
deposited both prior to cell assembling and during cycling. The
decomposition of electrolyte additives may also provide a
protection against defects formed during electrode handling or
during cell operation. Such defects ordinarily allow local
undesirable water decomposition in some portion of the electrode,
leading to self-discharge, gas generation, and cell degradation.
The decomposition of the electrolyte additives may "heal" such
defects and allow long-term cycle stability to be achieved.
[0090] The coating layer(s) on the electrode surface may be
deposited by one or more vapor deposition technique(s), electroless
deposition, electrodeposition, dip coating, sol-gel, or other known
methods of conformal deposition of coatings.
[0091] In some applications, an overall coating thickness (not
counting the pH-modifying moieties, if present) in the range of
about 5 nm to about 500 nm may be advantageous. Thinner coating may
be prone to defects. Thicker coatings may impede ion transport or
contribute to a significant portion of the total mass or volume,
which is undesirable.
[0092] In some applications, it may be advantageous for the
protective coating to gradually change in composition. In this
case, the internal stresses during cycling may be reduced and
delamination of the coating prevented.
[0093] In some applications, it may be advantageous for the
protective coating to contain micropores or mesopores. The presence
of such pores may enhance the stability range of aqueous
electrolytes. In addition, such pores may accommodate some of the
volume changes within the active material particles, thus
stabilizing the mechanical integrity of the electrode during
cycling.
[0094] Many intercalation-type active materials are compatible with
aqueous Li-ion batteries. Examples of such materials include but
are not limited to various layered oxide(s), spinel(s), and
olivines, to name a few. These include but are not limited to
lithium cobalt oxide, LCO, lithium manganese oxide, LMO, lithium
nickel manganese cobalt oxide, NMC, lithium iron phosphate, LFP,
various other lithium phosphates and fluorophosphates, various
lithium metal silicates, and many others. At the same time, many
conversion-type active materials offer higher volumetric Li
capacities than intercalation compounds. In addition, some of them
exhibit a specific Li insertion/extraction potential, which may be
advantageous for some applications. They are, however, mostly
incompatible with aqueous electrolyte solutions because they either
(at least partially) react with water or even (at least partially)
dissolve in water (in some stage of charge or discharge). Examples
of conversion-type active materials include but are not limited to
selenium, lithium selenide, sulfur, lithium sulfide, various metal
fluorides (such as copper fluoride, nickel fluoride, iron fluoride,
cobalt fluoride, and others), various metal chlorides, various
metal bromides, various metal tellurides, various oxides, various
nitrides, various phosphides, sulfides, various antimonides, and
others. Some other intercalation-type electrodes may similarly
exhibit undesirable reactions with aqueous electrolytes, but offer
advantages for some applications of aqueous Li-ion cells. Examples
of such advantages include a favorable Li insertion/extraction
potential, high volumetric or gravimetric capacity, or a high Li
insertion rate.
[0095] In order to overcome the incompatibility of some favorable
active materials with aqueous electrolytes, it may be advantageous
in some applications to enclose them in one or more Li-ion
permeable, but solvent impermeable protective shell(s).
[0096] FIG. 12 is a cross-section view of different example
particle designs incorporating one or more Li-ion permeable, but
solvent impermeable protective shell(s). As shown, each of the
example composite core-shell nanoparticles shown here is generally
composed of a Li.sub.2S core 1202 and a protective shell 1204 that
is permeable to Li ions, but not permeable to H.sub.2O. In some
particle designs, the core may further include carbon nanoparticles
1206 to enhance electrical conductivity. In some particle designs,
the core may further include a carbon matrix 1208 to enhance
electrical conductivity. In some particle designs, the shell may be
formed with a gradually changing composition 1210 as discussed
above. In some particle designs, the core may further include a
porous scaffolding matrix 1212 to enhance electrical conductivity,
as well as mechanical stability.
[0097] In some applications (e.g., when the shells are electrically
conductive), it may be advantageous for such shells to be deposited
on individual particles prior to electrode assembling. In other
applications (e.g., when the shells are electrically insulative or
when the shells could be damaged during electrode processing), it
may be advantageous for such shells to be deposited after the
electrode assembling. In yet other applications, it may be
advantageous to deposit the shells both times, before and
additionally after electrode assembling, for example to ensure the
lack of water-permeable defects or weak points within shells.
[0098] The use of many conversion-type active materials (such as
metal fluorides, sulfur, selenium, lithium sulfide, or lithium
selenide, as a few examples) in aqueous Li-ion battery cells has
been conventionally impractical because of their reactivity with
(or solubility in) water. However, the above core-shell structure
applied to such particles (where shell(s) around the particles
prevent water access to the conversion-type active material) may
provide unique capabilities to such Li-ion aqueous cells.
[0099] Examples of the electrically conductive, Li-ion permeable
and water impermeable shell materials include but are not limited
to graphitic, disordered, or amorphous carbon. In some cases, it
may be advantageous to use various metals (such as copper, nickel,
or iron, to name a few) or various metal alloys as conductive
coatings. It may be important, however, to make sure that the
deposited metals are further protected against corrosion. It may be
further important to make sure that the metal-coated electrodes are
not exposed to potentials where undesirable phase transformation
may take place. In some applications, it may be advantageous to use
conductive polymers (such as polyaniline, for example) as a shell
material.
[0100] Examples of electrically insulative shell materials include
various oxides (such as aluminum oxide, zirconium oxide, silicon
oxide, or various mixed oxides), various fluorides, various
sulfides, various mixed ceramics, various polymers, various
composites, and others. It may be important to make sure that the
electrode is not exposed to the potential where undesirable phase
transformation takes place. For example, titantium oxide should not
be exposed to a potential below around 1.7 V vs. Li/Li+. It may
also be important to make sure that the shell is compatible with
the electrolyte employed (e.g., so that it does not dissolve in the
electrolyte).
[0101] Similar to the protective shell(s) deposited for the purpose
of preventing aqueous electrolyte decomposition, the shells
deposited to protect the active material from undesirable reactions
with water may contain multiple layers. These layers may similarly
offer different functions. For example, in addition to protecting
the active material from unfavorable interactions with aqueous
electrolytes, these shells may provide one or more of the following
functions: (i) enhance electrical connectivity between individual
active particles; (ii) improve mechanical stability of the active
particles; (iii) reduce volume changes within the active particles
during cycling; and/or (iv) prevent aqueous electrolyte
decomposition at extreme potentials (such as oxygen generation at a
high potential of a cathode and hydrogen generation at a low
potential of an anode).
[0102] As discussed above, one layer may, for example, assist in
electrically connecting active particles of the electrode. In this
case, the layer should be electrically conductive. Examples of
materials for such a layer include but are not limited to a
conductive carbon coating or a conductive metal coating, which
should be stable in the potential range for the electrode of
interest. Nickel is an example of such a metal suitable for some
anodes. Aluminum is an example of such a metal suitable for some
cathodes. A layer can also be deposited in order to assist in
forming uniform coating of a subsequent (e.g., second) layer.
Examples of materials for such a layer include but are not limited
to metal(s), metal alloy(s), metal oxide(s), metal fluoride(s),
metal sulfide(s), various other ceramic coatings, polymer(s), and
composite(s), to name a few. It may be important that this material
should also be stable in the potential range for the electrode of
interest and not undergo undesirable phase transformation
reactions. As discussed above, a layer can also be deposited in
order to enhance the mechanical properties of the overall coating
or enhance the mechanical stability of individual particles.
Examples of materials for such a layer include but are not limited
to carbon, metal(s), metal alloy(s), metal oxide(s), metal
fluoride(s), various other ceramic coating(s), and composite(s), to
name a few.
[0103] In some embodiments, active cathode particles comprising a
conversion-type active material may be used in combination with
anode active particles comprising an intercalation-type active
material in a construction of aqueous Li-ion cells. In other
applications, an intercalation-type active material can be used in
the cathode and a conversion-type active material in the anode. In
yet other applications, it may be advantageous to use
conversion-type active materials for both electrodes or
intercalation-type active materials for both electrodes. In still
other applications, it may be advantageous to use both types of Li
storing materials (intercalation and conversion) in one electrode
(for example, when a high capacity conversion-type active material
residing in the core of an active particle is surrounded by a lower
capacity intercalation-type active material shell that stores Li
ions and simultaneously protects the core from unfavorable
interactions with an aqueous electrolyte).
[0104] All layers with a shell should be permeable to ion transport
in order to provide energy storage capabilities to active
particles.
[0105] In some applications, an overall thickness of the protective
shell in the range of about 5 nm to about 500 nm may be
advantageous. Thinner shells may be prone to defects. Thicker
coatings may impede the ion transport or contribute to a
significant portion of the total mass or volume, which is
undesirable.
[0106] In some applications, it may be advantageous for the
protective coating to gradually change in composition. In this
case, the internal stresses during cycling may be reduced and
delamination of the coating may be prevented.
[0107] In some applications, it may be advantageous for the
conformal coating(s) on the electrode surface to both (i) protect
some of the active material from reaction with the aqueous
electrolyte and (ii) impede or prevent decomposition of the aqueous
electrolyte at extreme electrode potentials (that is, prevent
oxygen generation on the cathode surface or hydrogen generation on
the anode surface). Methods described above may be used to produce
pH-regulating layers on the surface of such shells to enhance the
aqueous stability range. Similarly, other described methods may be
used to deposit layers of electrically insulative (yet Li-ion
permeable) material on the surface of such shells to further
enhance the stability range of an aqueous electrolyte.
[0108] Various deposition techniques may be used for the conformal
formation of layers or complete shells for various implementations
described above (such as preventing electrolyte decomposition or
preventing various undesirable reactions between the electrolyte
and active material, to name a few). Examples include but are not
limited to various vapor deposition techniques (such as chemical
vapor deposition or CVD, atomic layer deposition or ALD,
plasma-enhanced CVD, and plasma enhanced ALD, to name a few),
various wet chemistry deposition techniques (such as layer-by-layer
deposition, dip coating, solution precipitation, sol-gel,
electroless deposition, and electro-deposition, to name a few) and
other known techniques for the deposition of conformal layers on
porous electrode substrates or particles.
[0109] For example, for the formation of a nickel metal coating, a
CVD method may be used that involves thermal decomposition of a
Nickel-biscyclopentadienyl (Nickelocene, Ni(C.sub.5H.sub.5).sub.2,
or NiCp.sub.2) precursor or nickel-carbonyl (Ni(CO).sub.4)
precursor at elevated temperatures (for example, within a
temperature range of about 180-250.degree. C.). In some
applications (e.g., when a high degree of uniformity is required),
it may be advantageous to conduct CVD at reduced pressures (e.g.,
under vacuum). For the formation of a carbon coating (if the core
is thermally stable), a suitable polymer layer may be deposited on
the surface of the particles (for example, by a solution
precipitation method) and carbonized by annealing at elevated
temperature (e.g., above about 400.degree. C.). Alternatively, a
CVD method may be employed that involves decomposition of
hydrocarbons (such as acetylene) in a gaseous phase at elevated
temperature (e.g., above 400.degree. C.). A combination of such
methods can also be employed.
[0110] FIG. 13 provides an example of a high capacity aqueous
Li-ion battery with a pH-modified anode and cathode. Active cathode
particles that comprise one of the common intercalation-type Li ion
storing materials (such as lithium cobalt oxide, LCO, lithium
manganese oxide, LMO, or lithium nickel manganese cobalt oxide,
NMC) are used in this example cathode embodiment of Li-ion aqueous
cells. In some cases (for example, when active particles are
designed to have small volume changes during cycling and when their
surface is protected from direct interactions with water, as
previously described) active anode particles may comprise
conversion-type active material(s). In the current example, the
anode comprises environmentally-friendly low-cost sulfur (S)-based
core-shell particles that may offer over two times higher
volumetric capacity than the graphite currently used in
conventional organic Li-ion cells. While some conventional designs
have utilized S or Li.sub.2S-comprising active material within a
cathode (positive electrode) of a Li-ion or Li cell with an organic
or ionic liquid electrolyte, the use of a shell-protected S or
Li.sub.2S-comprising active material as an anode material with an
aqueous electrolyte is unique.
[0111] Many high capacity active material exhibit significant
volume changes during insertion and extraction of Li ions. Such
volume changes may induce defects in the functional conformal
coatings previously described. Such defects may lead either to the
undesirable reaction(s) of the aqueous electrolyte with active
material or induce decomposition of the aqueous electrolyte, or
both. It is therefore desirable for active particles as a whole to
have relatively small volume changes during cycling, and to use
such lower volume change particles in the construction of
electrodes for aqueous Li-ion cells with enhanced cell voltage.
[0112] Accordingly, in various embodiments, each of the active
material particles may include internal pores configured to
accommodate volume changes in the active material during the
storing and releasing of ions. When the active material is a high
capacity material that changes volume by more than about 10% during
insertion and extraction of ions (e.g., Li.sup.+, Na.sup.+, or
Mg.sup.2+ ions), the internal porosity of the active particles can
be used to accommodate these volume changes so that
charge/discharge cycles do not cause failure of the
particle/protective layer interface, and do not induce formation of
cracks in the protective layer(s). The overall porosity can be
optimized to maximize the volumetric capacity, while avoiding the
critical stresses that cause rapid composite failure or fatigue
during battery cycling. In some applications, when a relatively
brittle protective layer(s) is used or when the interface between
the electrode particles and the protective layer(s) is relatively
weak, then the presence of internal pores may prove to be
beneficial even when active material changes volume by less than
10%.
[0113] Such porous particles may be produced by a so-called
"bottom-up" approach, where the particles are built from smaller
building blocks. One example to produce such porous active
particles is utilization of an emulsion route. For example, active
material in the form of nanoparticles can be dispersed in the
suitable liquid. Binder (monomer or polymer) to keep the active
nanoparticles together can be added to the liquid as well. Another
type of additive (conductive particles, for example) can be
dispersed jointly with the active material nanoparticles. Then, the
suspension of the active particles with the binder may be
emulsified in a second liquid immiscible with the first. The size
of the porous particle may be controlled by the size of emulsion
droplets. The droplets of the emulsion may then be solidified by
solvent evaporation or monomer polymerization, yielding porous
particles containing pores. In yet another example, porous
particles may be produce by a so-called "balling" method, according
to which smaller (for example, nanosize) particles are agglomerated
together using a binder, which can be removed at later stages or
transformed into a solid (e.g., a solid carbon, by carbonization of
organic binders). In some examples, the particles can be further
annealed in a controlled environment to induce sintering of
individual nanoparticles. Another general route to produce such
particles is a "top-down" approach where pores are induced in solid
particles. In one example, the porous particles can be produced by
first forming two or more compound-comprising particles, where one
compound is leached out by dissolution or vaporization. In yet
another example, porous particles may be produced by partial
etching of solid particles.
[0114] In some embodiments, it may be advantageous for the active
particles with internal porosity and volume-changing active
material to be a composite of (i) a conductive material that does
not exhibit volume changes (or exhibits very low volume changes)
and (ii) volume-changing active material. In some cases, it may be
further advantageous for the "low volume change" material to
provide a rigid scaffold with internal pores partially filled with
a volume changing material. This architecture of the particles
allows one to further minimize the volume changes in such composite
particles during cycling. Conductive carbon is an example of a
material that may be used for such a scaffold.
[0115] FIG. 14 provides an example of different porous particle
designs containing a conversion-type active material (sulfur) that
experiences volume changes upon Li insertion. As shown, the
composite core-shell nanoparticles in this example are generally
composed of a porous sulfur core 1402 and a protective shell 1404
permeable to Li ions, but not permeable to H.sub.2O. In some
designs, the core may further include a porous scaffolding matrix
1406 to enhance electrical conductivity, as well as mechanical
stability. In some designs, the shell may be formed with a
gradually changing composition 1408 as discussed above.
[0116] In some embodiments, it may be advantageous for the
thickness of the features of the porous scaffold material to be
small, e.g., in the range of about 0.3 to about 50 nm in size.
Defective fragments of graphene (single or multi-layered with a
thickness in the range from 0.3 to 50 nm, for example), activated
carbon, carbon nanotubes, graphite ribbons, carbon fibers, carbon
black, dendritic carbon particles, and various other carbon
particles may serve as a scaffold material in some
applications.
[0117] In some embodiments, it may be advantageous for the porous
composite particles to be a nano-composite.
[0118] In some embodiments, it may be advantageous for the pores
within the active particles to remain small, e.g., in the range of
about 0.4 to about 10 nm.
[0119] In some embodiments, it may be advantageous for the "nodes"
of the active material deposited within the scaffold to be small,
e.g., in the range of about 0.5 to about 100 nm in size.
[0120] In some embodiments, it may be advantageous for the porous
active material (or for the "nodes" of the active material
deposited within the scaffold) to contain a secondary protective
coating. In this case, if the conformal coating around the
particles fails, this secondary coating may provide additional
protection against undesirable side reactions with the
electrolyte.
[0121] In some embodiments, conformal shells around the porous
composite particles may serve to prevent volume changes in the
porous particles. In some applications, it may be advantageous for
the shell to have gradually changing porosity or gradually changing
composition, or both (for example, to minimize stresses occurring
during battery cycling and improve stability of the shell-core
interface). It may further be advantageous for the shell to
gradually emerge from the porous core, again to minimize internal
stresses and improve mechanical stability of the composite active
particles.
[0122] The high rate capability of an aqueous electrolyte can
reduce the overall heating caused during use. In addition, high
temperature performance will not cause significant irreversible
degradation in an aqueous, pH-neutral Li-ion electrolyte. As such,
battery structures provided herein require little or no cooling
system. Because of the inherent safety of the cell, conventional
packaging used to make battery modules and packs can be reduced, as
they are no longer needed to serve the same protective role.
Instead, the battery module and packs can be used (e.g., in
electric vehicle applications) to protect passengers and absorb the
energy of impact in the case of a severe crash (the electrolyte is
safe). This may further improve the system-level performance of the
provided energy storage solution based on a pH neutral
electrolyte.
[0123] FIG. 15 is a flow chart illustrating an example method of
fabricating a battery electrode composition comprising active
particles. As shown, the method 1500 may comprise, for example,
providing active material particles to store and release ions
during battery operation (block 1510) and electrically connecting
the active particles with a current collector (block 1520). A
conformal protective coating may then be formed on the electrode
surface in such a way that the electrode remains porous while all
(or at least a significant portion) of its open pore surface area
is covered with such a coating (block 1530).
[0124] For connecting the active particle together during the
electrode fabrication, the method may utilize a mixing process for
mixing the active particles with a binder and an annealing process
for annealing at an elevated temperature to cause solidification of
the bonded particles in a particular shape. In some embodiments,
the surface of the active particles may allow sintering particles
together at elevated temperatures and thus not require a binder. In
some embodiments, the surface coating of the active particles may
deform during sintering or electrode preparation (e.g., during
annealing or during application of a mechanical pressure) in such a
way as to have a significantly smaller coating thickness in the
areas where particles touch each other. This may be advantageous,
for example, when the coating is electrically isolative, because in
the particle-to-particle contact points a significantly thinner
coating may provide, for example, paths for electron transport (for
example, via quantum mechanical tunneling).
[0125] As previously discussed, in some embodiments, the coating or
shell has gradually changing composition. This may be achieved, for
example, by gradually changing the composition of the coating
precursor.
[0126] In contrast to traditional Li-ion batteries, aqueous Li-ion
cells can be manufactured in a small, commodity, cylindrical form
factor, which may be advantageous for electric vehicle
applications. For example, such a multi-cell battery can be
designed to have a shape that fits the space available, rather than
building the car around a large prismatic design. Small cylindrical
cells using steel casings can be used to provide tremendous
rigidity to the module and pack, and in turn carry loads normally
borne by the chassis. With traditional Li-ion cells, such an
approach would never be used, since damaging the cells in an
accident would lead to nearly certain thermal runaway. This
approach, however, is made feasible by the aqueous Li-ion cells
disclosed herein.
[0127] In some embodiments, it may be advantageous for the thicker
electrodes of aqueous Li-ion batteries to contain pores (for
example, pores perpendicular to the electrode surface) to provide
channels for faster Li-ion electrolyte diffusion through the
electrode. The pore width may range, for example, from as little as
about 20 nm to as much as about 500,000 nm (0.5 mm). This structure
of the porous electrode may be particularly advantageous if the
electrode thickness is in the range of about 0.2 mm to about 5 mm.
In this case, having the "channel" pores within the electrode may
significantly enhance the rate or power performance of such aqueous
Li-ion batteries.
[0128] In some embodiments, it may be advantageous to embed a
porous metal (e.g., a metal or conductive carbon foam or mesh)
current collector within the electrode. In this case, both
mechanical properties of the electrode and electrical conductivity
of the electrode will be enhanced. It is noted, however, that it
some embodiments (e.g., in cases when the metal current collector
does not exhibit high over-potential for water decomposition), it
may be advantageous to deposit a conformal protective coating on
all of the open internal surface area of the electrode, including
the current collector.
[0129] Compared to conventional Li-ion batteries, the dramatic cost
reduction of the provided aqueous Li ion technology also comes from
different manufacturing technology that could be enabled by the
significantly higher ionic conductivity of aqueous Li-ion
electrolytes. Because aqueous electrolytes offer higher
conductivity than those based on the carbonate solvents used in
commercial Li-ion cells, the electrodes can be made about 0.5-5
millimeters thick while maintaining acceptably high power
characteristics. This is because high electrical conductivity is
relatively easy to maintain and because relatively slow (e.g., less
than around "2C") charging rate in graphite anode-based commercial
Li-ion cells is limited by the low solid electrolyte interphase
stability, high charge-transfer resistance, and Li plating (due to
low lithiated graphite potential). All these factors disappear or
become greatly reduced (charge transfer resistance) in aqueous
Li-ion systems. As a result, with thick electrodes, bulk (molding)
rather than surface (coating) manufacturing methods may be used in
some embodiments of aqueous Li-ion batteries. In some applications,
it may be advantageous to use a process that is akin to alkaline
batteries rather than traditional Li-ion cells.
[0130] FIG. 16 shows a comparison of two cell constructions,
including a conventional Li-ion cell side by side an aqueous Li-ion
cell as described herein. A traditional Li-ion cell in a
cylindrical 18.times.65 mm case utilizes anywhere from 15 to 30
winds of a very thin electrode to occupy that volume. In order to
create the winding, great care is taken to cast the active material
onto thin copper and aluminum foils which are then sliced into
sections nearly three feet long, stacked with two separators, and
wound with extreme precision to ensure all edges are aligned. Any
misalignment or variation in the amount of active material along
the three-foot foil can lead to electrical short circuits and
thermal runaway. As a result, these processes require extremely
high precision and many additional quality control steps which
result in a relatively high cost of assembly.
[0131] There are also technical limitations in this process. For
example, the minimum thickness of Cu and Al that must be used to
keep from tearing during assembly is approximately 10 .mu.m. Much
of this foil, however, is unnecessary from an electrical
conductivity standpoint, adding little to the performance of the
cell other than allowing for robust assembly. The copper and
aluminum conductors in a cell make up 5 g of a 45 g cell or about
11% of the total mass. The separator, while light, takes up 7% of
the volume. The case adds 12-14% by volume and 10% by mass. Much of
this is essentially dead weight, as well as dead volume and
unnecessary cost, which are compared below.
[0132] This construction methodology leaves only 60-65% of volume
available for the functional active electrodes in the cell. The
reason for this complexity and inefficiency stems directly from the
need to keep electrode thicknesses at or below 100 .mu.m to allow
sufficient ionic conductivity in the electrode during operation.
The need for electric vehicles, for example, to operate at low
temperatures exaggerates these limitations even further, as the
ionic conductivity of the commercial organic electrolytes often
drops tenfold when operating at -20.degree. C. Finally, due to the
high sensitivity of cell performance to moisture residues,
extensive drying and expensive glovebox-operated electrolyte
filling/sealing protocols must be employed.
[0133] In contrast, assembly for the provided aqueous Li ion
technology is dramatically simpler. As in alkaline cells, a
cylindrical pellet of anode material may be prepared, typically
about 0.5-8 mm thick depending on the diameter of the battery, and
inserted into the casing from the open top end. The pellet is
electrically conducting and free standing, and makes contact with
the casing, which serves as a current collector and negative
terminal for the cell. Next a cylindrical separator is inserted,
after which a cylindrical cathode pellet, followed by the addition
of the electrolyte, the top cap, and the positive electrode pin
(which occupies the same space and doubles up functionally for the
traditional central vent tube). Once firmly pressed, the cell is
crimped in a manner similar to conventional cells.
[0134] Unlike conventional Li-ion cells, however, the entire
process can take place in a humid environment and does not require
the construction of expensive dry rooms. The simple construction is
not only cheaper and faster to manufacture, but carries additional
safety benefits and enhanced process robustness. In traditional
Li-ion construction, the separator spans nearly three feet, and two
layers are required for the winding. As a result, engineers have
pushed the separator to be as thin as possible to minimize its
inactive volume--anywhere from 16-25 .mu.m in typical cells. This,
however, reduces the safety of the cell, as the thinner separators
are more susceptible to internal short circuits due to defects,
particulate contaminants, and dendrites. A penetration through the
separator during charging is a common cause of sudden thermal
runaway in Li-ion systems. To combat the problem, automotive cells
use thicker separators--typically, 25 .mu.m and thicker--but this
reduces the energy density of their cells and increases the $/kWh
cell costs. In the construction provided herein, however, the
separator length is less than about 1/20.sup.th of that in a
conventional cell, and can therefore be made thicker to improve
safety and eliminate unwanted internal short circuits with minimal
impact on cost or energy density.
[0135] In contrast to traditional alkaline cells, in some
embodiments, it may be advantageous to use more than one positive
or more than one negative electrode in the construction of the
aqueous Li-ion cells. In this case, the thickness of each electrode
may be kept relatively small (for example, about 0.2-1 mm), while
the overall power performance may be high, allowing fast charging
(within an hour or faster) in cells with a relatively large
diameter of more than 10 mm.
[0136] In some embodiments, it may be advantageous to produce
planar cells, instead of cylindrical cells. In this case, cells may
be packed together more efficiently, providing less "free volume"
space between individual cells.
[0137] FIG. 17 shows select performance characteristics of the two
cell constructions, including a conventional Li-ion cell side by
side an aqueous Li-ion cell as described herein. Deconstruction of
a mass-produced, 2.9 Ah, 3.6 V traditional Li-ion cell showed the
anode and cathode capacity with a volumetric capacity to be 400 and
600 mAh/cc, respectively. Because certain example embodiments may
utilize a similar, traditional cathode with a surface modification
technique, they may also reach 600 mAh/cc in well-designed cells.
Capacity of pure Li.sub.2S is 1,931 mAh/cc. Conservatively assuming
that 48% of the volume will be occupied by the non-active
components and pores, it can be estimated that the protected
S-based anode capacity may approach 1,000 mAh/cc for this example
of an aqueous Li-ion cell. Since a different manufacturing
technology can be employed for the fabrication of aqueous Li-ion
cells, the volume occupied by the separator may be reduced, and the
Al and Cu foils may be eliminated. As a result, for an
18650-volume-equivalent aqueous Li-ion cell with such 1000 mAh/cc
anode and 600 mAh/cc cathode, it may be estimated that a 5.3 Ah
capacity may be achieved, along with an average voltage of, for
example, 1.9 V, and an energy density of 610 Wh/L (200 Wh/kg). This
is around 90% of traditional high energy Li-ion cells, but at
substantially lower cost.
[0138] The forgoing description is provided to enable any person
skilled in the art to make or use embodiments of the present
invention. It will be appreciated, however, that the present
invention is not limited to the particular formulations, process
steps, and materials disclosed herein, as various modifications to
these embodiments will be readily apparent to those skilled in the
art. That is, the generic principles defined herein may be applied
to other embodiments without departing from the spirit or scope of
the invention.
* * * * *